FAST C/C++ Implementation of the Karatsuba Multiplication algorithm. This is the only C++ implementation that I found online that uses straight C++ primitives to store data instead of std::vector or std::string objects. Because of this, it's pretty speedy. You can use this file in your program

KaratsubaMultiplication.cpp

//============================================================================
// Name        : KaratsubaMultiplication.cpp
// Author      : Evin Ugur (http://evinugur.com)
// Version     : 1.0
// Copyright   : Copyright 2015. You can use this code however and wherever you want no strings attached
// Description : C++ Functions to Perform Karatsbuba Multiplications
//============================================================================

#include <iostream>
#include <math.h>
#include "KaratsubaMultiplication.h"
using namespace std;



int getLength(long long value) {
	int counter = 0;
	while (value != 0) {
		counter++;
		value /= 10;
	}
	return counter;
}

long long multiply(long long x, long long y) {
	int xLength = getLength(x);
	int yLength = getLength(y);

	// the bigger of the two lengths
	int N = (int)(fmax(xLength, yLength));

	// if the max length is small it's faster to just flat out multiply the two nums
	if (N < 10)
		return x * y;

	//max length divided and rounded up
	N = (N/2) + (N%2);

	long long multiplier = pow(10, N);

	long long b = x/multiplier;
	long long a = x - (b * multiplier);
	long long d = y / multiplier;
	long long c = y - (d * N);

	long long z0 = multiply(a,c);
	long long z1 = multiply(a + b, c + d);
	long long z2 = multiply(b, d);


    return z0 + ((z1 - z0 - z2) * multiplier) + (z2 * (long long)(pow(10, 2 * N)));

}


// C++ example (uses cout...)
// (this code works in straight C too though if you use a different main function)
int main() {
	// two numbers
	long long a = 2406994;
	long long b = 2328563;
	cout << multiply(a,b) << endl;
	return 0;
}

KaratsubaMultiplication.h

/*
 * KaratsubaMultiplication.h
 *
 *  Created on: Feb 4, 2015
 *      Author: Evin Ugur (http://evinugur.com)
 */

#ifndef KARATSUBAMULTIPLICATION_H_
#define KARATSUBAMULTIPLICATION_H_

int getLength(long long value); // returns the number of digits a number has
long long multiply(long long x, long long y); // Karatsuba multiplication of two numbers



#endif /* KARATSUBAMULTIPLICATION_H_ */

In line 33, it should be $N==1$. Because you have got the length of value $x$.